The present invention is directed to a device for cutting paper webs. A water jet device is used to cut the paper webs.
Paper web cutting devices are employed, for example, in high-speed web-fed printing presses which presses may be used for printing illustrated and other printed products with large numbers of copies. The cutting devices are used to divide a wide web, on which several pages of the printed products have been printed side-by-side, into partial webs, each of which partial web corresponds to a single page. The partial webs separated in this way are bundled and are then fed to the folding apparatus.
A device for cutting a moving web by use of a water jet is known from U.S. Pat. No. 4,266,112, but the questions of what material the web can be made of is left open. The suitability of this device for cutting a paper web cannot be determined from this publication because one skilled in the art, knowing that paper has the tendency to absorb water and to then swell, would assume that this will also occur if this device is used for cutting paper. Further than that, because of the low web speed of less than 1 m/s, this device is not suitable for being employed in connection with printing presses.
A device for a shaping processing of paper in a printing press is known from U.S. Pat. No. 5,797,320. This device uses a water jet for cutting out or perforating designs on a printed sheet.
The structure of a jet for generating a high-pressure water jet for cutting materials is known from U.S. Pat. No. 5,730,358.
A device for use in the trimming of the edges of paper webs with the aid of a water jet is known from WO 97/11814 A1. This device is employed in the course of paper production. In this case, it must be assumed that the paper to be trimmed contains residual moisture, so that moisture possibly picked up from the cutting jet is not noted in an interfering manner. No great demands are being made on the accuracy of the cutting. In particular, no accurate register is required, because the paper to be cut has not yet been imprinted.
DE 91 03 749 U1 discloses a device for cutting paper webs by use of a water jet cutting device.
Technische Rundschau [Technical Magazine], No. 18, 05/08/1973, pp. 25, 27, 29, 31 describes cutting parameters for various materials.
The object of the present invention is directed to providing a device for cutting paper webs.
In accordance with the present invention, this object is attained by the use of a water jet cutting device that can be arranged in a web-fed rotary printing press between a printing unit and the inlet of the folding apparatus. The distance of the nozzle to the paper web can be selected to optimize cutting of the web. Water pressure of greater than 3500 bar is preferably used.
The advantages which can be obtained by the present invention reside, in particular, in that it is possible to arrange such a device in a space-saving manner at any arbitrary straight section of the path of the paper web. In the vicinity close to the paper web, the subject invention requires only such sufficient installation length as corresponds to the dimensions of the jet nozzles of the cutting device.
In contrast to the device for cutting paper webs in accordance with the present invention, known cutting devices for use in printing presses include rotating so-called upper and lower cutters, between which the paper web is passed. One of these cutters also functions as a deflection roller for the paper web. In the course of operating these prior cutters, it is necessary to make absolutely sure that their circumferential speed corresponds to the running speed of the paper web to be cut, so that they do not exert braking or acceleration forces on the paper web, which forces, at the high web speeds of modern printing presses, can easily result in tearing of the paper web. Regular maintenance of these cutters is required in order to assure that the paper web is actually cleanly cut at all times and is not being torn by dull or badly aligned blades. Therefore, the blades must be accessible to the maintenance personnel, and they must be replaceable. It is thus necessary to provide access to the place where these prior cutters are installed, which has the result that the processing section, consisting of printing press, device for cutting, and folding apparatus requires considerable space.
A further advantage of the device for cutting paper webs in accordance with the present invention requires little maintenance in comparison with traditional cutter arrangements.
A further advantage of the subject invention is that dust, which might possibly be created in the course of cutting the paper, is substantially carried along by the water jet, so that it, in a manner different from a cutting device consisting of an upper and lower cutter, substantially occurs only on one side of the paper web and for this reason alone can be more easily caught. An aspiration of the dust active in the immediate vicinity of the paper web is no longer required. The suction hoods, which previously had been used for aspirating the dust and which have extended over the entire width of the web at the respective locations of the cutters and increased the space requirements of the cutting device and which made maintenance of the cutter additionally more difficult, are not required by the device for cutting paper webs in accordance with the present invention.
The distance between the cutting jet and the paper web preferably corresponds to three to ten times the sonic running time transversely in respect to the jet diameter. It is presumed that the high-speed jet generated by the nozzle passes through three phases on its path; a first one, in which it forms a coherent jet, a second, in which the coherent jet disintegrates as a result of coarse drops, and a third phase, in which the coarse drops again disintegrate and form fine droplets. In the first phase, the jet is well suited for cutting homogeneous media. In the second phase, in which the individual drops exert an intermittent force on the material to be cut, the jet is particularly suited for cutting media having an interior structure, such as grainy mineral materials, or stacks of paper with a layer structure.
While the disintegration of the jet into fine droplets probably is the result of the slowing down of the jet by air, the transition of the jet from the first phase into the second phase is a result of its surface tension. From the point of view of surface tension, or surface energy, a fine jet of constant diameter represents an unstable equilibrium. Minimal deviations of the diameter tend to grow, so that the jet is constricted and disintegrates into individual drops. The velocity with which the constriction takes place is necessarily proportional to the velocity with which pressure effects are propagated in the jet, i.e. to the speed of sound in the jet. Cutting experiments have shown that the transition from the first phase to the second phase must take place at a distance D1 from the nozzle corresponding to three to ten times the sonic running time transversely in respect to the jet, i.e. D1=3xc2x7cxc2x7d less than vxc2x710xc2x7cxc2x7d, where c=speed of sound in water, d=jet diameter, v=jet velocity, and D1=distance from the jet to the paper web.
Too short work distances D1, in particular of less than three times the sonic running time, are less preferred. It is presumed that the reason for this lies in the velocity distribution of the water transversely to the jet direction. As long as the water moves through the nozzle, the flow velocity in the center of the nozzle bore is considerably greater than at the edge of the nozzle bore, where the water is slowed down because of friction because of contact with the walls of the bore. This velocity. distribution is initially maintained, even when friction ceases when the jet exits from the bore. Only after a certain minimum path is the velocity distribution in the jet homogenized to the extent necessary so that the edge areas of the jet will also provide a dependable total cutting effect.
A collecting receptacle for the cutting jet is arranged on an opposite side of the web at a distance of preferably 5 to 15 mm from the web. Such a distance is short enough to dependably collect the jet after the jet has passed through the web and is sufficiently large to prevent the web from being pushed against the collecting receptacle in the case of possible fluctuations in the tension of the web.
The collecting receptacle should be constructed in such a way that it dependably slows the cutting jet down, while at the same time preventing water spray from exiting the collecting receptacle in the direction toward the web. The jet must not accomplish its cutting effect in the collecting receptacle. To this end, the collecting receptacle is provided with at least one deflecting surface, that is arranged obliquely to the jet direction, and is used for slowing the jet down. With such an oblique arrangement, only the velocity component of the jet which extends vertically in relation to the deflecting surface can exert a cutting effect. This component is proportional to the cosine of the angle which the jet forms with the normal surface extension of the deflecting surface.
A further feature or aspect which contributes to the limitation of the cutting effect of the water jet on the collecting receptacle lies in selecting the distance between the jet and the deflecting surface of the collecting receptacle so that the jet arrives at the deflecting surface when the jet is in its third phase.
In accordance with a preferred embodiment of the present invention, the cutting device is comprised of several jets arranged in a row for the simultaneous cutting of a web into a large number of partial webs. In this case, it is practical for the collecting receptacle to extend along the row for use in collecting the jets from all of the nozzles.
To dependably prevent the exit of spray from the collecting receptacle and, if required, to draw off water which was atomized during cutting, it is preferred that the collecting receptacle can be evacuated or connected to a vacuum source.
The device for cutting paper webs in accordance with the present invention can also be used for the cutting of webs consisting of several layers of paper placed on top of each other. In such a case, the distance between the jet and the web should preferably be selected to lie in the upper range of the spacing intervals mentioned above, in order to assure that the jet impinges on the web while the jet is in its second phase.